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Drug Interactions: Pharmacodynamic Interactions
Definition:
Pharmacodynamic drug interactions occur when two or more drugs affect each other’s action at the level of the drug target (receptors, enzymes, physiological systems), without changing their concentrations in the body.

Types of Pharmacodynamic Interactions
1. Additive Effect
The combined effect equals the sum of the effects of each drug taken separately (1 + 1 = 2).
Example:
Alcohol + benzodiazepines: Both depress CNS → enhanced sedation.
Aspirin + paracetamol: Both lower fever/add to pain relief.
2. Synergistic Effect
The effect of two drugs together is greater than the sum of the individual effects (1 + 1 greater than 2).
Example:
Sulfamethoxazole + trimethoprim: Two antibiotics that together strongly inhibit bacterial growth.
Levodopa + carbidopa: Carbidopa boosts levodopa’s CNS effect.
3. Antagonistic Effect
One drug reduces or opposes the effect of another (1 + 1 less than 2 or can even = 0).
Example:
Naloxone + morphine: Naloxone blocks opioid receptors, reversing morphine’s effect.
Beta blockers + beta agonists (e.g., salbutamol): Beta blocker reduces bronchodilator effect.
Mechanisms of Pharmacodynamic Interactions
At the Same Receptor

Two drugs act as agonist and antagonist on the same receptor (e.g., morphine and naloxone at opioid receptors).
At Different Receptors with Similar or Opposing Physiological Effects

Both drugs may influence the same organ/function by different mechanisms.
Example: Nitrates (vasodilators) + sildenafil (PDE5 inhibitor): Both lower blood pressure, can cause severe hypotension.
With a Common Clinical Outcome

Example: Multiple CNS depressants (antihistamines, alcohol, benzodiazepines)—different mechanisms but additive CNS depression.

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